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Psyche 
[March 
a defensive mechanism against insect and vertebrate herbivores, 
pathogens, and perhaps competitors. This view has been supported, 
as far as insects are concerned, by Dethier (1954, 1970), Thorstein- 
son (i960), Ehrlich and Raven (1965), Jermy (1966), Whittaker 
and Feeny (1971), and Erickson and Feeny (1973). Much of the 
interaction between insects and these secondary substances is sensory 
in nature, and such substances may inhibit or deter feeding (Thor- 
steinson i960, Gill and Lewis 1971), or may prove toxic to non- 
adapted larvae (Taylor 1959). Some plant species even synthesize 
and accumulate sterols which mimic insect molting secretions (Wil- 
liams 1970). These phyto-ecdysones have been found in numerous 
fern species and prevent molting in insects feeding on such species 
(Whittaker and Feeny 1971). 
These secondary chemicals are not the only chemical means that 
plants have evolved to protect themselves from insect attack. Plant 
proteins are often deficient in some individual amino acids necessary 
for insect growth and development (Lord 1968, Boyd 1970). The 
nutritional quality or adequacy of the food plant is of utmost im- 
portance to a phytophagous insect (Friend 1958, Legay 1958). 
Aphids (Auclair et al. 1957), beetles (Allen and Selman 1955, 
1957), butterflies and moths (Hovanitz and Chang 1962, Feeny 
1970), grasshoppers (House 1959, Dadd 1961), flies (Chapman 
1969) , and other insects (Gilmour 1961, Levinson 1962), have all 
been shown to exhibit quite variable feeding responses which in turn 
influence larval development, mortality, fecundity and fertility, when 
reared on different host plant species or on the same host plants 
grown under differing conditions or ages. Gordon (1959) has 
suggested that nutrient deficiencies in plants “. . . may be a result 
of natural selection of inedibility.” The interaction between the 
nutritive adequacy of leaves and secondary chemistry has been dem- 
onstrated for oak trees and oak leaf tannins (Feeny 1968, 1969, 
1970) . 
Agriculturists and plant breeders have become aware of the prob- 
lems that increased yield and palatability of crop species present in 
terms of the plant’s inherent defensive mechanisms (Snelling 1941, 
Painter 1951, Allard i960, Briggs and Knowles 1967). The oldest 
record of inherent plant resistance to insect herbivores was by 
Havens (1792) in which he recognized the Hession fly resistance 
of the Underhill variety of wheat. However, scarcely 200 papers 
have dealt with this subject in the 148 year period from 1792 to 
1940 (Snelling 1941). Generally, as plants species were domesti- 
cated and cultivated, various morphological and physiological changes 
